A storage system for providing a host computer with a storage area for storing data includes a number of physical disks such as hard disks for storing data. The storage system uses a RAID (Redundant Array of Independent (or Inexpensive) Disks) technology to make storage areas of the multiple physical disks redundant, and form a RAID group. Furthermore, the storage system creates a logical volume from a part of the RAID group as a storage area with a capacity demanded by the host computer, and provides the host computer with the created logical volume.
As for the storage system, there is a thin provisioning technology. In the thin provisioning, the logical volume is not provided to the host computer as a storage area with a fixed capacity. Instead, in this technology, a virtual logical volume (virtual volume, hereinafter the “VVol”) is provided to the host computer, and in response to processing to write data into the storage system from the host computer and the like, a storage area using a segment as a unit is assigned to the virtual volume from a storage area (volume pool) created from multiple logical volumes. There has been a storage system capable of dynamically extending a storage capacity to be provided to a host computer by use of the thin provisioning technology (for example, Patent Citation 1).
Note that the segment is a storage area created by dividing the logical volume (Pool Volume, hereinafter the “PoolVol”) included in the volume pool into areas each having an appropriate capacity by using a logical block address (hereinafter, the “LBA”) or the like. Here, the LBA is an address used for specifying a position in the logical volume when the host computer reads and writes data from and into the storage system.
Moreover, there has been known the following technique (for example, Patent Citation 2). Specifically, in two storage systems (a storage system A and a storage system B) which are connected via a data communication network such as a SAN (Storage Area Network), a logical volume included in the storage system A is associated with an external volume that is a virtual volume created in and by the storage system B. Thus, the logical volume in the storage system A is incorporated by the storage system B, and the incorporated logical volume is provided to a host computer and the like as a volume in the storage system B (hereinafter referred to as “external connection”).
The use of the external connection technique can extend a capacity of the storage system B incorporating the logical volume. Moreover, since the storage system B that has incorporated the logical volume provides the host computer with the logical volume for the host computer, management of the storage systems is facilitated.    Patent Citation 1: Japanese Patent Application Laid-open Publication No. 2003-15915    Patent Citation 2: Japanese Patent Application Laid-open Publication No. Hei 10-283272